I had combo interview last Wednesday for both i751 and N400 in Detroit. MI. I want to share my experience with you guys. Please feel free to reach out if you have any questions.   So, a little bit of background, I applied for i751 (ROC) in January 2018 and had received two extension letters so far without other updates (one for 12-month and one for 18-month). The case status of my i751 says "case was received". I was a little bit inpatient about it and decided to apply for N400 (